Casual Sex or Wedding Bells?

Opposites attract, as the saying goes. But a new study suggests this adage may only hold for one-night stands, not long-term relationships.

Decades of research indicate that people tend to trust those with faces similar to their own and are even more likely to marry them. Evolutionary theory provides an explanation for the first tendency: we are wired to recognize and help our kin because they are likely to share our genes. But evolution should also protect us against the dangers of inbreeding by preventing us from hopping in the sack with close relatives.

To find out how these conflicting motivations play out, Lisa DeBruine, a psychologist at St. Andrews and Aberdeen Universities, Scotland, presented 144 college students with 36 pairs of computer-generated faces. The faces in each pair were of the same race and the opposite sex as the viewer, but one was manipulated to have facial features more similar to those of the viewer.

As in earlier studies, the students found faces more trust-worthy if they were like their own. When it came to the prospect of a one-night stand, students found facial similarity a turn-off, DeBruine reports online today in the Proceedings of the Royal Society B: Biological Sciences. She concludes that people are less sexually attracted to individuals who look like kin in order to avoid inbreeding.

So why would spouses tend to resemble each other? That's still a mystery, but DeBruine guesses that marrying a distant--but still familiar-looking--relative might translate to a more cohesive extended family, and thus more help with child-rearing. The next step is to compare the faces of people in real relationships, both short-term and long-term, says DeBruine. "I would expect short-term partners to look significantly less alike."

"This is good science" and not just "idle speculation," says Ian Penton-Voak, a psychologist at the University of Bristol in the United Kingdom. It helps rule out the idea that a person's choice of mate is guided by a single rule such as avoiding mating with someone who seems related, he says. Instead, the results support the view that mate choice has become increasingly complex during our social evolution, allowing us to make context-specific decisions about other people.